The invention relates to a control device for a continuously variable transmission (CVT) that is based on the dual-piston principle.
The control of a continuously variable transmission based on the dual-piston principle is known, e.g., from the publication DE 195 46 293 A1. Continuously variable transmissions are cone-pulley transmissions that can be shifted within a continuous (step-free) range of transmission ratios. They have pairs of conical disks, i.e., one pair each on the input and output side of the transmission, with an endless chain-belt device making a torque-transmitting connection between the pairs of conical disks. More specifically, the continuously variable transmission according to FIG. 1 essentially consists of a disk pair SSA that is rotationally locked to a torque-input shaft 20 and a disk pair SSB that is rotationally locked to an output shaft 21 of the transmission. Each of the two disk pairs comprises an axially displaceable disk and an axially fixed disk. An endless chain-belt device 22 transmits torque from the disk pair of one shaft to the disk pair of the other.
The disk pair SSA on the input shaft 20 can be axially tightened against the chain-belt device 22 by a first piston/cylinder unit 23. In analogous manner, the disk pair SSB on the output shaft 21 can be axially tightened against the chain-belt device 22 by a second piston/cylinder unit 24.
Third and fourth piston/cylinder units 25 and 26, serving to shift the ratio of the transmission, are arranged to work in parallel, respectively, with the first piston/cylinder unit 23 on the input shaft 20 and the second piston/cylinder unit 24 on the output shaft 21. The desired transmission ratio is set or changed by simultaneously adding pressure fluid to one and removing pressure fluid from the other of the pressure chambers of the third and fourth piston/cylinder units 25 and 26, respectively. This is accomplished by connecting the pressure chambers either to a pressure-medium pump or to a drain conduit as needed in each case. In other words, a change in the transmission ratio is effected by adding pressure medium to one chamber and thereby expanding its volume while, at the same time, draining the other chamber at least partially of pressure medium and thereby reducing its volume. The respective pressurizing and draining of the pressure chambers occurs through a valve 1 as illustrated, e.g., also in FIG. 3 of the aforementioned publication DE 195 46 293 A1. The valve 1 has different ports, of which the port 2 is connected to a pressure-fluid pump (not shown). Port 3 of valve 1 is connected to the oil sump or reservoir tank. The third and fourth piston/cylinder units 25, 26 of the disk pairs SSA, SSB are connected to the valve 1 through ports 4 and 5 (conduits L1 and L2). Port 6 serves to control the valve 1 by means of a biasing pressure in pressure chamber 7. The biasing pressure in pressure chamber 7 can be governed by a proportional valve (not shown).
The slide piston 8 of the valve 1 can be configured with a smaller cross-section in a portion 9 and a larger cross-section in a portion 10. The corresponding bore widths inside the valve housing are dimensioned accordingly, i.e., differently for the respective portions 9 and 10 of the slide piston 8. In addition, the slide piston 8 can have a portion with an axial channel 11 that has a radially directed opening at a location 12. An internal piston 13 is arranged so that it can move inside the axial channel 11.
A plurality of forces are acting on the slide piston 8 and, according to their sum total, can produce a resultant force that pushes the slide piston 8 either to the right or to the left. The individual forces are symbolized in FIG. 1 by the arrows F6, F4, F5 and F14.
Directed to the right and represented by arrow F6 is a force that is proportionate to both the pressure at port 6 (thus also inside the pressure chamber 7) and the cross-sectional area of the portion 9 of the slide piston 8.
Also directed to the right and represented by arrow F4 is a force that is proportionate to both the pressure at port 4 and the difference between the cross-sectional areas of the portions 9 and 10.
Directed to the left and represented by arrow F5 is a force that is nearly proportionate to the pressure at port 5 and the cross-sectional area of the axial channel 11. The pressure at port 5 communicates with the axial channel 11 through the radial opening 12. The radial opening 12 could also be designed as a hydraulic resistance element for damping the motion of the slide piston. The pressure acting in the axial channel 11 by way of the radial opening 12 is nearly the same as at port 5. This pressure exerts a rightward push against the internal piston 13 which, in turn, bears against the plug 16. The same pressure, acting on an effective area equal to the cross-section of the axial channel 11, also exerts a leftward push on the slide piston 8.
A further leftward-directed force, symbolized by arrow F14, is generated by spring 14 exerting a leftward push on the slide piston 8 and also bearing against the plug 16.
FIG. 1 shows the valve 1 in a state where the slide piston is in its midway position. The force F6 is about equal to the force F14. The portion 8a of the slide piston 8 closes off the port 2 leading to the pressure-medium pump.
Via the shutter edges 15 and 15xe2x80x2, port 5 and port 4 are connected to port 3. Given that port 3 has a connection to the oil sump, the respective pressures at ports 5 and 4 are nearly equal and very small. Consequently, the forces F5 and F4, which have opposite directions and nearly cancel each other, are likewise very small. With the pressure being equal at ports 4 and 5, no resultant displacing force is applied to the piston/cylinder units 25, 26 through the conduits L1, L2.
If the force F6 is greater than the force F14, slide piston 8 will move to the right. The connection between ports 5 and 3 is interrupted. Port 2 becomes connected to port 5. An in-flow of pressure medium occurs, whereby the pressure at port 5 is increased. At the same time, the shutter edge 15xe2x80x2 opens the connection from port 4 to port 3 and thus to the oil sump. This allows the pressure medium to escape to the sump. Consequently, the pressure at port 4, and thus the force F4, is small, i.e., nearly zero. As the pressure rises at port 5, the force F5 will keep increasing up to the point where the force F5 is equal to the difference between the forces F6 and F14 (F6 minus F14). As soon as this is the case, the slide piston 8 will stop its rightward travel. If the pressure at port 5 and, consequently, the force F5 continues to increase, the slide piston 8 will move to the left until the connection between ports 2 and 5 is interrupted and the further pressure rise is blocked. Ports 5 and 3 become connected, and the passage stays open until the pressure at port 5 has decreased to the point where the force F5 is again equal to the difference between the forces F6 and F14.
This process, which is appropriately termed pressure balancing, regulates the pressure at port 5 automatically to an amount of proportionate magnitude as the difference between the forces F6 and F14.
If the pressure at port 5 is too high, fluid is drained off as the shutter edge 15 opens the connection between ports 5 and 3, while the in-flow connection between ports 2 and 5 is blocked. If the pressure at port 5 is too low, the in-flow connection between ports 2 and 5 opens and the drain connection between ports 5 and 3 becomes closed off.
The pressure at port 5 acts on the piston/cylinder unit 26 by way of conduit L2. Conduit L1, along with the piston/cylinder unit 25 is nearly pressure-free. As a result, the loop radius at which the endless chain-belt device 22 runs on the disk pair SSB is increased, while the loop radius of the chain-belt device 22 at the disk pair SSA is decreased. The result is a transmission-ratio shift to a slower speed.
If the force F6 becomes smaller than the force F14, slide piston 8 will move to the left as a result. The connection between ports 4 and 3 is interrupted and port 2 becomes connected to port 4. An in-flow of pressure medium occurs, whereby the pressure at port 4 is increased. At the same time, the shutter edge 15 opens the connection from port 5 to port 3 and thus to the oil sump. This allows the pressure medium to escape to the sump. Consequently, the pressure at port 5, and thus the force F5, is small, i.e., nearly zero. As the pressure rises at port 4, the force F4 will keep increasing up to the point where the force F4 is equal to the difference between the forces F14 and F6 (F14 minus F6). As soon as this is the case, the slide piston 8 will stop its leftward travel. If the pressure at port 4 and, consequently, the force F4 continues to increase, the slide piston 8 will move to the right until the connection between ports 2 and 4 is interrupted and the further pressure rise is blocked. Ports 4 and 3 become connected, and the passage stays open until the pressure at port 4 has decreased to the point where the force F4 is again equal to the difference between the forces F14 and F6. Again, a process of pressure balancing is taking place, regulating the pressure at port 4 automatically to an amount of proportionate magnitude as the difference between the forces F14 and F6. If the pressure at port 4 is too high, fluid is drained off as the shutter edge 15xe2x80x2 opens the connection between ports 4 and 3, while the in-flow connection between ports 2 and 4 is blocked. If the pressure at port 4 is too low, the in-flow connection between ports 2 and 4 opens and the fluid-draining connection between ports 4 and 3 becomes closed off. The pressure at port 4 acts on the piston/cylinder 25 unit by way of conduit L1. Conduit L2, along with the piston/cylinder unit 26 is nearly pressure-free. As a result, the loop radius at which the endless chain-belt device 22 runs on the disk pair SSA is increased, while the loop radius of the chain-belt device 22 at the disk pair SSB is decreased. The result is a transmission-ratio shift to a faster speed.
Known from DE 195 46 293 is a torque sensor that serves to generate a load-dependent (more specifically, torque-dependent) belt-tightening pressure in a continuously variable transmission. It is also known to configure a torque sensor of this kind as a valve which, through the displacement of an axially movable part of the torque sensor, closes off the connection to the reservoir tank and thereby controls the pressure in the pressure chamber in accordance with the desired amount of torque to be transmitted.
The object of the present invention is to provide an improved control device for a continuously variable transmission (CVT). Specifically, the transmission to be controlled has a first disk pair SSA that is rotationally locked to an input shaft and a second disk pair SSB that is rotationally locked to an output shaft. Each of the two disk pairs has an axially movable desk and an axially fixed disk. An endless chain-belt device transmits torque between the disk pairs SSA and SSB. First and second piston/cylinder units are associated with the first and second disk pairs SSA and SSB, respectively, to produce the compressive forces that tighten the disk pairs against the chain belt. Further, third and fourth piston/cylinder units, serving to shift the ratio of the transmission, are associated with the first and second disk pairs SSA and SSB, respectively. The transmission ratio is shifted by simultaneously adding pressure fluid to one and removing pressure fluid from the other of the third and fourth piston/cylinder units.
The control device according to the invention is a system of hydraulic valves in which a pressure-reducing valve is used to produce the belt-tightening pressure for the first and second piston/cylinder units, while the function of shifting the transmission ratio is performed by a ratio-shifting valve device that adds pressure medium to one and simultaneously removes pressure medium from the other of the third and fourth piston/cylinder units. In particular, the invention provides that the pressure-reducing valve and the ratio-shifting valve device work in a cascade arrangement where an offset pressure valve directs the pump-circulated pressure medium first to the pressure-reducing valve to produce the belt-tightening pressure and subsequently, but only after the belt-tightening pressure has been established, to the ratio-shifting valve device.
The arrangement of a pressure cascade where the assurance of a sufficient amount of belt-tightening pressure takes precedence over the ratio-shifting function represents an essential advantage of the invention. The pressure-reducing valve that produces the belt-tightening pressure can be a torque sensor of the kind that is described in the German patent application DE 198 12 033 A1 which, by reference, is hereby expressly incorporated in the present disclosure. Giving first priority to assuring the required level of belt-tightening pressure protects against the risk of chain-belt slippage even in case of a possible leak in the belt-tightening system. The ratio-shifting function is performed only after the required level of belt-tightening pressure has been reached. In the possible case of higher than normal leakages in the hydraulic system, this means that the shifting function has to be performed with a reduced supply of pressure medium, i.e., ratio-shifting will be slower. If the priorities were reversed, i.e., if the ratio-shifting function had first priority, the transmission could suffer damage if ratios were shifted too fast, because the torque sensor generating the belt-tightening force would receive no pressure fluid and, therefore, the chain belt would slip. Thus, if the ratio-shifting function were given precedence over the belt-tightening function, special measures would be necessary to put limits on how fast the ratio could be shifted and thereby prevent slippage, taking the possibility of additional leakages into account.
In an advantageous embodiment of the invention, the offset pressure valve has a slide piston on which a counter-force, e.g., the force of a compression spring pushing against the slide piston, and the belt-tightening pressure together hold equilibrium against the pump pressure. Thus, the pump pressure that is present at the connected input ports of the offset pressure valve and the pressure-reducing valve is regulated at a pressure level that exceeds the belt-tightening pressure produced by the pressure-reducing valve by at least an amount of offset pressure.
In the foregoing arrangement, it is advantageous to provide a check valve in the conduit that connects the input ports of the offset pressure valve and the pressure-reducing valve. The check valve is oriented so that it is held open and allows the passage of pressure medium when the pressure at the input port of the offset pressure valve is higher than at the input port of the pressure-reducing valve.
Returning to an advantageous concept mentioned above, the pressure-reducing valve can be provided in the form of a torque sensor with a pressure compartment that is pressurized with fluid supplied by a pump. The torque sensor is arranged in the torque-flow path between a torque-input part and a torque-output part so that the torque sensor itself transmits at least part of the torque that passes from the input part to the output part. Connected to the pressure compartment is a torque-sensor valve with at least two parts that can move in relation to each other and thereby control the pressure in the pressure compartment which, in turn, controls the torque-transmitting capacity of the transmission.
In a further advantageous embodiment of the inventive device, the input ports of the pressure-reducing valve and of the offset pressure valve are connected. The pressure-reducing valve has a second port connected to the first and second piston/cylinder units, and it can also have a drain port. In particular, the pressure-reducing valve has a slide piston on which a biasing force holds equilibrium against a counterforce, e.g., a spring force, and the belt-tightening pressure that exists at the second port of the pressure-reducing valve, so that the pressure-reducing valve regulates the belt-tightening pressure as a function of the biasing force. The second port of the pressure-reducing valve is connected to a second input port of the first offset pressure valve, which admits the belt-tightening pressure to the slide piston of the first offset pressure valve. The input ports of the pressure-reducing valve and the offset pressure valve are connected so that the pressure at these ports is regulated at a level that exceeds the belt-tightening pressure by at least an offset pressure.
A variation of the foregoing embodiment is functionally identical in all of the features disclosed except for the configuration of the pressure-reducing valve which, in this case, is designed so that the biasing force and the belt-tightening pressure together hold equilibrium against the counterforce. As in the preceding embodiment, the pressure-reducing valve regulates the belt-tightening pressure as a function of the biasing force.
In combination with any of the foregoing embodiments, it is advantageous if the part of the control device that is dedicated to the ratio-shifting function is a pressure-reducing valve device to which a second offset pressure valve (VSV2) is assigned. The second offset pressure valve functions as a logic OR-gate and regulates the fluid pressure at the input port of the ratio-shifting valve at a level that exceeds the greater of the pressures existing at either of two output ports of the ratio-shifting valve by at least an amount of offset pressure.
In advantageous embodiments of the invention, the ratio-shifting valve device of the foregoing description consists of either a single valve unit or a plurality, preferably a pair, of valve units. In embodiments of the device that use two valve units, the latter can preferably be controlled either by one common biasing force or by two separate biasing forces.
In all of the foregoing embodiments, any of the counterforces as well as biasing forces are generated preferably by mechanical, hydraulic or electrical means.
In a particularly favorable arrangement, the ratio-shifting valve device is constituted of two pressure-limiting valves (DBV1, DBV2) whose input ports are connected to the output port of the offset pressure valve (VSV). The output port of the first of the two pressure limiting valves (DBV1) is connected to the fourth piston/cylinder unit, i.e., to the second disk pair (SSB), while the output port of the second pressure limiting valve (DBV2) is connected to the third piston/cylinder unit, i.e., to the first disk pair (SSA). Each of the two pressure limiting valves has a slide piston. Within its respective valve housing, each slide piston will always seek a position where the forces acting on the slide piston are in equilibrium with each other. In each of the two pressure-limiting valves DBV1 and DBV2, the respective forces in equilibrium are the counterforce, the force generated by the pressure at the output, and a biasing force introduced through the pressure at a bias-pressure port of the respective pressure-limiting valve. Governed by the respective biasing forces, the first pressure limiting valve (DBV1) allows an outflow of pressure medium from its output port to the drain while the second pressure limiting valve (DBV2) directs an inflow of pressure medium from its input port to its output port, and vice versa. At the cross-over point where the pressure limiting valves (DBV1, DBV2) reverse their respective flow directions, the regulation is such that it will generate approximately the same, preferably low pressure levels at the output ports of the pressure-limiting valves.
Advantageous versions of the preceding embodiment use either a common biasing force controlling both the first and second pressure limiting valve or, alternatively, two separate biasing forces for the first and second pressure limiting valves, respectively. Further in the preceding embodiment, the one or more biasing forces as well as the counterforces can be mechanically, hydraulically and electrically generated forces.
Instead of using two pressure-limiting valves, several advantageous embodiments will now be described in which the functions of the two pressure-limiting valves (DBV1, DBV2) of the ratio-shifting valve device are incorporated in a single pressure-limiting valve unit with
an input port that is connected to the output port of the offset pressure valve,
first and second output ports that are connected to the fourth and third piston/cylinder units, respectively,
third and fourth output ports connected to a drain,
a slide piston subjected to a counterforce, a pressure force caused by a pressure at the first output port working against a pressure at the second output port, and a biasing force.
In a first advantageous embodiment of a single pressure-limiting valve unit, the pressure at the first output port is communicated through first radial and axial passages in the slide piston to a first cylinder compartment containing a first internal piston seated against the valve housing. Analogously, the pressure at the second output port is communicated through second radial and axial passages in the slide piston to a second cylinder compartment containing a second internal piston seated against the valve housing. In this first embodiment of a single pressure-limiting valve unit, it is advantageous to provide a mechanical retaining device that holds the second internal piston essentially in an axially fixed position in relation to the valve housing.
In a second advantageous embodiment of a single pressure-limiting valve unit, the pressure at the first output port acts against the surface area of a first step of the slide piston, and the pressure at the second output port acts against the surface area of a second step of the slide piston.
In a third advantageous embodiment of a single pressure-limiting valve unit, the pressure at the first output port is communicated through a passage opening in the piston to an axial channel in the slide piston containing an internal piston seated against the valve housing, while the pressure at the second output port acts against the surface area of a step of the slide piston.
In each of the three preceding embodiments, but with the bias-control function suitably modified, the first and second output ports could be exchanged so that the first output port leads to the third piston/cylinder unit and the second output port leads to the fourth piston/cylinder unit.
In any of the preceding embodiments of single pressure-limiting valve units, the counterforce and the biasing force can be generated mechanically, hydraulically, or electrically.
In any of the embodiments of the present invention, it is advantageous if the ratio-shifting valve device has shutter edges performing a pressure-regulating function and other shutter edges performing a switching function between a first state where pressure regulation takes place at the first output port while the second output port is connected to the drain, and a second state where pressure regulation takes place at the second output port while the first output port is connected to the drain. As a part of the same advantageous concept, when the ratio-shifting valve device is in a midway condition, the first and second output port are both connected to the drain.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both in its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiment with reference to the accompanying drawing.